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Reading the Persona of a Process

Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files sys/types.h and unistd.h.

uid_t Data Type
This is an integer data type used to represent user IDs. In the GNU library, this is an alias for unsigned int.

gid_t Data Type
This is an integer data type used to represent group IDs. In the GNU library, this is an alias for unsigned int.

uid_t getuid (void) Function
The getuid function returns the real user ID of the process.

gid_t getgid (void) Function
The getgid function returns the real group ID of the process.

uid_t geteuid (void) Function
The geteuid function returns the effective user ID of the process.

gid_t getegid (void) Function
The getegid function returns the effective group ID of the process.

int getgroups (int count, gid_t *groups) Function
The getgroups function is used to inquire about the supplementary group IDs of the process. Up to count of these group IDs are stored in the array groups; the return value from the function is the number of group IDs actually stored. If count is smaller than the total number of supplementary group IDs, then getgroups returns a value of -1 and errno is set to EINVAL.

If count is zero, then getgroups just returns the total number of supplementary group IDs. On systems that do not support supplementary groups, this will always be zero.

Here's how to use getgroups to read all the supplementary group IDs: 

gid_t *
read_all_groups (void)
{
  int ngroups = getgroups (0, NULL);
  gid_t *groups
    = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
  int val = getgroups (ngroups, groups);
  if (val < 0)
    {
      free (groups);
      return NULL;
    }
  return groups;
}